THE ASIMOYA
SCIENTIFIC NAME: Asimina triloba
FAMILY: Annonaceae
Asimina triloba, often called the American pawpaw, was a favourite food of the native Indians and an essential part of the diet of the early European settlers. It was harvested from wild patches and little cultivation of this fruit has occurred since then.
A few commercial endeavours and some variety selection have taken place, but this work has generally been forgotten or lost as the enthusiasts have died and their properties have been sold. Several enthusiasts championed them at the beginning of the century, learning how to grow them and selecting superior types. I know that at least 21 cultivars were named. However, most of these, as far as I know, have been either lost or exist only in private gardens.
Even in the natural state, many selections were good enough for commercial production. But the fact that they were originally so easy to obtain from the bush, meant few people bothered to grow them themselves. Nevertheless, it is a crop we in New Zealand should not discard without testing its worth.
Confused Naming
A browse through American magazines is most frustrating. Their references to pawpaws turn out to be not the pawpaw we know, that tropical melon-shaped fruit, Carica papaya, belonging to the plant family Caricaceae. Instead, they mean a completely different fruit, the Asimina triloba. This fruit was known to the Indians as 'Arsimin', and has also been variously referred to as the Hardy Custard Apple and the Kentucky Banana, which are equally confusing as names. The Asimina triloba actually belongs to the family Annonaceae, the same plant family which contains our cherimoya (Annona cherimola) and atemoya (Annona cherimola X A. squamosa).
Because of its close relationship with these other fruits, we are recommending the name 'Asimoya' as our new name for the Asimina triloba. It is a pleasant-sounding name, with the advantage that it avoids all confusion with the pawpaw, Carica papaya.
Some facts
Asimina triloba is basically a diploid, 2n = 18, but some stands contain triploids. Seedlings of these stands can be in the ratio of 40: 1. Triploids would probably be seedless. If the crop developed, there would also be a place for someone to produce tetraploids from the better types.
As far as I know, there are none of the superior selections in New Zealand. I don't even know that they still exist in the USA, but hopefully some still do. Seed was introduced 10 or so years ago into New Zealand and although only a few of the seedlings survived, some are now beginning to fruit. Like the ones I visited recently in Hamilton. Many, however, have grown slowly and haven't set fruit yet. I have heard of none that have fruit worth writing home about.
Some General Information
The tree, as seen in New Zealand, is very similar to a small cherimoya but is reported to be much hardier in cold winter conditions. They often sucker in the wild, so that a single plant may cover a large area called a "patch" (as in the children's song "Way down yonder in the Pawpaw Patch"). However, there were large specimen trees, with one champion having a girth of 65cm, a height of 16 m and a spread of 18 m.
Growth, from most reports, seems very slow for the first two seasons, unless ideal conditions prevail. But there are two diverse schools of thought about ideal conditions for young trees. One is that they need about 50 - 85% shade and the other is that they need really good sunlight. I tend toward the idea that they need shade for at least two years.
Selected Superior Varieties in the USA
Little Rose (a pollinator), Davis, Kurle, Glaser, Sweet Alice, Vena, Rees, Over Eese, Uncle Tom.Late Varieties: Gable, Tiedke, Jumbo, Shannondale, Osborne, Buckman, Martin, Taylor No.1 and No. 2.
Early Varieties: Fairchild (considered the best), Ketter(er) (2nd best), Hopes August.
Many of these are names of early enthusiasts. If you discover any of these make sure they are preserved, and I would love to hear about them.
Fruit
There are two basic types of fruit white-fleshed and yellow-fleshed. The white-fleshed type is generally considered late maturing, with a mild to flat (even insipid) flavour.
The yellow-fleshed type generally ripens earlier and has a rich flavour, some being too rich but some delicious. These are the ones we want. The white one is also a longer banana shape and the yellow a fatter oval.
Large fruit will weigh 250-340 grams, and are 8-13 cm long and 2-4 cm wide. Smaller fruit will weigh 86- 200 grams.
Skin colour is green, turning brown to black when ripe. The flesh is green, turning white or yellow (even pinkish) as they ripen. When the fruit is ripe, the flesh goes buttery and the odour sweetens. They have a high sugar content, 16% sucrose (similar to a banana) and 17% carbohydrate. The flavour varies according to soil type and fertility.
The approximate fruit composition is: Flesh 50- 80%, Seeds 14-17%, Skin 6-10%.
Climate and Soil Types
In the U.S.A. the asimoya (see how easily we slip into using the new name) grow from Northern Florida to Ontario, and from Nebraska to Texas. However, the main areas are Pennsylvania, Ohio and Indiana. It is evident therefore, that there is a lot of plasticity in the asimoya germplasm for adaption to various climates. The best types so far, however, have come from Susquehanna, Potomac and Ohio.
The general climate requirement is considered to be for winter chilling to break bud dormancy, followed by a hot summer with between 150-260 growing days. A rainfall of 75cm in the growing season, or supplementary irrigation, is needed. Soil can be from clay to sandy loam but soils rich in humus are needed for regular good fruit production - lots of mulch, decayed vegetable matter, along with plentiful lime and potash.
Cropping
Good cropping will initially need hand-pollination. This seems evident from the number of trees in New Zealand that are flowering but not fruiting. Current information is unclear whether all varieties are self fertile, or whether they need crossing. A little trial and error should sort that out. Given the variability in other characteristics, there are probably both types.
Hand-pollination is very easy. Petals can be removed to make it easier. The main difficulty is getting fresh pollen when the female stigmas are receptive. Also, a gentle touch is needed, as the style breaks easily. So use a light touch with a soft camel hair brush. Well -grown trees should begin cropping after five years. Yields of 50-100 fruit per tree are possible. As local trees so far are setting 10 or less fruit per tree, we have a long way to go.
Harvesting
The harvesting season in New Zealand will probably be between February and May. This will depend on whether we have early or late types. Pick fruit as they start to soften. The colour may also change. Store in a cool place. The old method was to store them wrapped in oat straw. Most fruit kept in a fridge should be ready to eat in two weeks. Longer storing types may take longer.
Propagation
Ideally seed should be kept damp in sphagnum or something similar. Cold treatment in a fridge for 8 weeks and then soaking in water for 24 hours before sowing works well. Cover seed with at least 2 cm of soil. Sowing can be direct into the field with 5 to 6 seeds per hill.
Cuttings take relatively easily from softwood to semi-softwood. The size of the cutting can vary considerably from 1 cm to 30 cm long. Where possible, use bottom heat and mist. Less sophisticated methods will also work reasonably well.
Grafting is best done from October to early November.
Transplanting into the Field
Asimoyas are tricky things to transplant. Many are lost at this stage. The best time to do this is probably between August and September. Shading and irrigation will be worth trying, to minimise losses at this stage.
Conclusion
In New Zealand attempts to grow asimoya have so far been slow and not very productive. This may be, to a large extent, due to our lack of knowledge of what we are doing, plus the fact we are growing seedlings rather than superior selection. I feel sure we haven't seen the true potential of this crop yet.
What we need to do is locate and import varieties such as Fairchild and Ketter(er). Then learn how and where to grow them best. With our wonderful climate we tend to expect everything to be easy. With a little more effort we may see the asimoya's full potential.
This article has been based on my own observations, plus reference material from U.S.A. especially the Californian Rare Fruit Growers Handbook: Pawpaw Tree - Asimina triloba, Volume 6 (1974).
DATE: November 1996
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